Continuously recording mechanical relaxation spectrometer

ABSTRACT

The continuously recording mechanical relaxation spectrometer includes two bridge connected selsyn type motors operated by two a.c. voltages of different frequencies, one motor being a reference motor connected in reverse phase to the remaining driving motor. A specimen, the moduli of which are to be measured as a function of temperature or frequency, is subjected to torsional oscillation by the driving motor in a controlled temperature environment. Variations in the load on the driving motor are sensed as voltage variations across a load resistor connected to the bridge connected motor system, and these voltage variations are fed to a phase analyser which provides signals to operate one axis of an X-Y recorder. The remaining axis of the recorder is operated by signals from a temperature sensing unit positioned adjacent the specimen.

United States Patent 11 1 Hedvig et al.

[451 Sept. 4, 1973 CONTINUOUSLY RECORDING I MECHANICAL RELAXATIONSPECTROMETER [75] Inventors: Peter Hedvig; Laszl Miskolczy, both ofBudapest, Hungary [73] Assignee: Muanyagipari Kutato Intezet,

Budapest, Hungary 22 Filed: Apr. 5, 1971 21 Appl. No.: 128,620

[30] Foreign Application Priority Data Apr. 6 1970 Hungary 440 [52] US.Cl. 73/99, 73/l5.6 [51] Int. Cl. G0ln 3/32 [58] Field of Search 73/l5.6,99, 101; 318/172 [56] References Cited OTHER PUBLICATIONS MechanicalInvestigations of Elastomers in a Wide Range of Frequencies PhilippoffJoum. of Applied Physics, Vol. 24, No. 6, June, 1953 PrimaryExaminer.lerry W. Myracle Attorney-Young & Thompson [57] ABSTRACT Thecontinuously recording mechanical relaxation spectrometer includes twobridge connected selsyn type motors operated by'two a.c. voltages ofdifferent frequencies, one motor being a reference motor connected inreverse phase to the remaining driving motor. A specimen, the moduli ofwhich are to be measured as a function of temperature or frequency, issubjected to torsional oscillation by the driving motor in a controlledtemperature environment. Variations in the load on the driving motor aresensed as voltage variations across a load resistor connected to thebridge connected motor system, and these voltage variations are fed to aphase analyser which provides signals to operate one axis of an X-Yrecorder. The remaining axis of the recorder is operated by signals froma temperature sensing unit positioned adjacent the specimen.

6 Claims, 7 Drawing Figures GEiiERATOR PHASE ANALYZER- f/A TEMP.LPRQGRAM.

RECORDER I 4 Sheets-Sheet 1 Fig.2

"Patented Sept. 4, 1973 3,756,074

4 Sheets-Sheet 2 Fig.3 6" SC/S F igA Patented Sept. 4, 1973 4Sheets-Sheet 5 Fig.5

Fig.6

Patented Sept. 4, 1973 3,755,074

4 Sheets-Sheet 4 CONTINUOUSLY RECORDING MECHANICAL RELAXATIONSPECTROMETER The present invention relates to a continuously recordingmechanical relaxation spectrometer for testing the structure of plasticand rubber systems by continuous measurement of the real and imaginaryparts of the complex torsional modulus as a function of the temperature.

Particularly suited for testing the structure of plastic and rubbersystems are methods for measuring the effects of dynamic (periodic)stresses as a function of the temperature, or as a function of thefrequency. The changes of the bending and torsional moduli as a functionof the temperature are related to the molecular structure of thematerial. Depending on the nature of the material, these moduli exhibitcharacteristic resonances which are connected with the mobility of themolecules or parts of the molecules.

The methods known hitherto do not permit quick and continuous recordingof the mechanical relaxation spectrum. However, this is needed not onlyfor making the testing easier and quicker but also because the structureof viscoelastic materials changes irreversibly during heat treatment.These changes are from a practical viewpoint very important, and theycannot be investigated by the usual measuring equipment.

In one group of the known methods the specimen to be tested is subjectedto free torsional oscillations, and the oscillations are recordedoptically or by a digital counting device. With such an apparatus, therelaxation spectrum may be obtained by a lengthy calculation only. Therecording time of the free oscillations is rather long, thus the timefor measuring a spectrum even by using a computer takes at least tenhours.

In another group of the known methods, the substance to be tested isglued to a soft iron plate or a small magnetic plate is fixed on it inorder to induce oscillations electro-magnetically. Although this methodmakes continuous recording possible, the use of a foreign material(magnetic plate) and the uncertainty of the connection between themagnetic plate and the test specimen makes it difficult to use.Therefore below the vitrification temperatures this method is notsuitable for investigation of dispersional ranges.

A further group of the hitherto known solutions uses ultrasonicvibrations. With this solution a technical difficulty is encounteredfrom the severe requirements relating to dimensions of the specimen.Furthermore, by the ultrasonic method, measurements can be performedonly at high frequencies; while in case of plasties, the flow frequencyrange is of interest.

Finally, a group of the hitherto known solutions uses forcedoscillations excited electromagnetically. The detection, however, iscarried out in this case too by an optical method or by a complicatedelectrodynamic system. In the known electrodynamic devices, the moving(oscillating) system and the detecting coils are separated. This makesthe design complicated and utilization difficult. In addition, thehitherto known methods based on forced oscillations can be used onlywith great mechanical losses.

The aim of the invention is to ensure a solution enabling continuous andquick recording of both components of the complex torsional modulus overa wide temperature range and within a wide range of mechanical losses.

The invention is based on the following recognitions:

l. The mechanical relaxation spectrum may quickly and continuously berecorded as a function of the temperature by using a singlephase-reversing selsyn-type motor, to the two pairs of coils of whicha.c. voltages of different frequencies are applied and to the shaft ofwhich the specimen to be tested'is connected. By a phase analysis of thedriving current of the soestablished electromechanical system, electricsignals proportional to the real part G and to the imaginary part G" ofthe complex modulus may be obtained.

2 The current components proportional to G and G" respectively under (1)may be separated by a twochannel phase-sensitive detector.

3. If for producing the mechanical oscillation, two a.c. voltages withrelatively high frequencies are used, as in (l), the separationaccording to phase of the current required by the electromechanicalsystem becomes possible at a significantly higher frequency than themeasuring frequency, rendering detection very simple and sensitive.

4, Compensating for losses originating from the bearings becomespossible by using two identical electro mechanical systems in such awaythat the currents required by the systems should be out of phase.

Thus an essentially higher level of sensitivity can be achieved than bythe methods known hitherto based on forced oscillations.

The invention thus relates to a continuously recording mechanicalrelaxation spectrometer for measuring the temperature dependence of thetorsional modulus of plastic and rubber systems, comprising an X-Yrecorder and an electronic temperature controller. According to theinvention, the spectrometer incorporates a phase-sensitive selsyn-typemotor, operated by two a.c. voltages of different frequencies, and anelectronic phase analyzer.

The apparatus according to the invention has in a preferred embodiment agas flow-type temperature programming device for continuous temperaturevariation.

In a further preferred embodiment of the apparatus according to theinvention, two bridge-connected selsyn-type motors are provided.

In still another preferred embodiment of the invention, a two-channelphase analyzer and a two-channel X-Y recorder for recordingsimultaneously the real and imaginary parts respectively on the complextorsional modulus are provided.

These and other objects, features and advantages of the invention willbe readily apparent upon a consideration of the following specificationand claims taken in conjunction with the accompanying drawings, inwhich:

FIG. 1 is a block diagram of the recording mechanical relaxationspectrometer of the present invention;

FIG. 2 is a curve disclosing the mechanical relaxation spectrum ofpolytetrafluoroethylene;

FIG. 3 is a curve disclosing the mechanical dispersion and absorptionspectrum of polyvinyl chloride;

FIG. 4 is a curve showing the mechanical relaxation spectrum ofpolymethyl-methacrylate;

FIG. 5 is a curve showing the mechanical relaxation spectra of twodifferent commercial polyethylene specimens;

FIG. 6 is a curve showing the mechanical relaxation spectrum of a lowpressure-type I-ID polyethylene; and

FIG. 7 is a curve showing the mechanical relaxation spectrum ofpolyethylene subjected to heat treatment and exposed to electronirradiation.

Referring now to FIG. I, from generator G, an a.c. voltage of 50 Hz, andfrom generator G, an a.c. voltage of frequency variable between 50 and IHz is applied to the appropriate pair of coils of the selsyn-type motorM To the shaft of the motor is connected the specimen m, in the form ofa rod, a plate or a bundle of fibers, which are placed in the gasthermostat T of known type, which may for example be a copper block inthe form of a heat exchanger through which nitrogen gas flows. Electricheater e heats the copper block to heat the nitrogen gas which keeps thetest specimen under a nitrogen atmsophere at a temperature that isvaried between -1 50 C. and .+200 C. by the temperature-programmingdevice HP at a given rate.

Into the circuit of generator G there is connected in reversed phase areference selsyn-type motor M Thus if the mechanical loads on the shaftsof both motors are equal, only a very small current will flow throughthe common load resistance R,,. When the mechanical characteristics ofthe specimen are changed, a voltage drop occurs across resistance R,,.This a.c. voltage, the amplitude and phase of which are determined bythe torsional moduli of the sample, is separated into two components bythe phase analyzer FA. By appropriate phase setting, it can be achievedthat the output voltage of the phase analyzer is proportional to thereal part G or to the imaginary part G" of the complex torsionalmodulus. The real part G is sometimes referred to in the literature asthe storage modulus and the imaginary part as the loss modulus. By usinga double-phase analyzer, the temperature dependence of both G and G maybe recorded simultaneously.

For recording the spectrum, an X-Y recorder (KI) is used, the X-axis ofwhich is operated by the copperconstantan thermocouples z,.placed nearthe specimen.

The principal advantages of the apparatus according to the invention areas follows:

I. It is simple, easy to operate, and automatic;

2. It ensures a quick and continuous measurement covering a wide rangeof temperatures l to l.5 hours, from -150 C. up to +200 C.);

3. Both the real and the imaginary components of the complex torsionalmodulus may be recorded continuously and simultaneously;

4. It enables measurement of spectra as a function of the temperature,at different, fixed, frequencies; and

5. It can be widely used for testing of plastic and rubber systems.

The principal fields of application of the equipment are as follows:

a. Routine qualification and production surveys of plastic and rubbersystems;

b. Testing of mixtures and copolymers with special regard to shockprooftough plastic systems;

c. Testing of structural changes during processing;

d. Testing of ageing processes;

e. Testing of cross-linking processes;

f. Testing the thermomechanical instabilities and irreversibilities; and

f. Testing the structure of complex plastic systems, e.g., glassfiber-reinforced polyester and cord-rubber systems and heavily loadedplastic and rubber systems.

The operation of the equipment according to the invention is shown inthe drawings.

In FIG. 2, the mechanical relaxation spectrum of polytetrafluoroethyleneis shown in the temperature range between l50 C. and +l50 C. Themeasurement of a polytetrafluoroethylene plate 1.5 mm. thick was usedand the temperature dependence of the imaginary part G" of the complextorsional modulus was recorded at 8 Hz frequency. The frequency of thegenerator G was in this case 50 Hz and that of generator 0, 58 Hz. Theobtained relaxation spectrum agrees well with spectra published in theliterature recorded by lengthy, point-by-point measurements. The timeneeded for obtaining the given spectrum was only one hour.

In FIG. 3, the mechanical dispersion spectra of l mm. thick p.v.c. isshown. The measurement was carried out at 8 Hz and the temperaturedependence of both components G and G" was recorded simultaneously.

In FIG. 4, the mechanical relaxation spectrum of polymethyI-methacrylateis shown in the T, range (between 20 C. and 150 C.). In this case thetemperature dependence of G" was measured at 8 1-12.

In FIG. 5, the mechanical relaxation spectra of two different commercialpolyethylene specimens are shown. One of the specimens, that of higherdensity, was produced by the low pressure process. The other, lowerdensity, Celene 600I-type specimen, was produced by the high pressureprocess.

To illustrate the advantages of the apparatus according to theinvention, the change of the mechanical relaxation spectrum of lowpressure-type I-ID polyethylene was measured after rapid cooling down to1 50 C. While cooling down, the crystallinity of polyethylene isreduced, causing the relaxational peak to be shifted toward lowertemperatures. The spectra are shown in FIG. 6.

Curve (1) relates to a specimen which was not heat treated, and curve(2) to a heat-treated (from C. abruptly cooled down to l50) specimen. Itis seen that by this method physical changes in the structure due toheat treatments can be easily investigated.

The relatively low crystallinity polyethylene subjected to heattreatment as described above was exposed to 1.8 MeV electronirradiation. The dose was 30 Mrad. As a consequence of the irradiation,the mechanical relaxation peak was shifted toward higher temperatures(FIG. 7). This phenomenon was interpreted as being due to the buildingup of a cross-linked structure within the polymer.

We claim:

1. A continuously recording mechanical relaxation spectrometer formeasuring the temperature dependence of the complex torsional modulus ofplastic and rubber systems, comprising a phase-sensitive selsyntypemotor having a pair of coils and a rotatable drive shaft, means forgenerating two a.c. voltages of different frequencies, means forapplying said voltages to said coils to drive said motor and to rotatesaid drive shaft, means connecting said drive shaft to a specimen undertest to apply torsion to said specimen upon rotation of said driveshaft, phase-sensitive output measuring meansconnected to said motor toprovide an output signal that varies with variations in the load placedon said motor by said specimen under test, means for measuring thetemperature of said specimen under test, and means for simultaneouslyrecording said temperature and said output signal.

2. A spectrometer as claimed in claim 1, said recording means comprisingan X-Y recorder having an X recording axis and a Y recording axis, saidtemperaturemeasuring means being connected to operate one axis of saidX-Y recorder and said output-measuring means being connected so thatsaid output signal operates the remaining axis of said X-Y recorder.

3. A spectrometer as claimed in claim 1, and heatin means to heat saidspecimen, said heating means including temperature-programming means forvarying the temperature of said specimen.

4. A spectrometer as claimed in claim 1, said phasesensitiveoutput-measuring means including a reference phase-sensitive selsyn-typemotor in bridge connection with the first-mentioned motor, saidreference motor being connected in reversed phase relation to saidfirst-mentioned motor.

5. A spectrometer as claimed in claim 4, said generating meanscomprising first-generating means connected to provide an ac. voltage ofa first frequency to said first-mentioned and reference motors andsecondgenerating means connected to provide a second ac. voltage of asecond frequency to said first-mentioned and reference motors, thesecond generating means being variable to provide different frequenciesto said first-mentioned and reference motors.

6. A spectrometer as claimed in claim 5, said outputmeasuring meanscomprising a load resistor connected between said first generating meansand said bridgeconnected first-mentioned and reference motors, and

phase-analyzer means connected to said load resistor. l l 1K l

1. A continuously recording mechanical relaxation spectrometer for measuring the temperature dependence of the complex torsional modulus of plastic and rubber systems, comprising a phasesensitive selsyn-type motor having a pair of coils and a rotatable drive shaft, means for generating two a.c. voltages of different frequencies, means for applying said voltages to said coils to drive said motor and to rotate said drive shaft, means connecting said drive shaft to a specimen under test to apply torsion to said specimen upon rotation of said drive shaft, phase-sensitive output measuring means connected to said motor to provide an output signal that varies with variations in the load placed on said motor by said specimen under test, means for measuring the temperature of said specimen under test, and means for simultaneously recording said temperature and said output signal.
 2. A spectrometer as claimed in claim 1, said recording means comprising an X-Y recorder having an X recording axis and a Y recording axis, said temperature-measuring means being connected to operate one axis of said X-Y recorder and said output-measuring means being connected so that said output signal operates the remaining axis of said X-Y recorder.
 3. A spectrometer as claimed in claim 1, and heating means to heat said specimen, said heating means including temperature-programming means for varying the temperature of said specimen.
 4. A spectrometer as claimed in claim 1, said phase-sensitive output-measuring means including a reference phase-sensitive selsyn-type motor in bridge connection with the first-mentioned motor, said reference motor being connected in reveRsed phase relation to said first-mentioned motor.
 5. A spectrometer as claimed in claim 4, said generating means comprising first-generating means connected to provide an a.c. voltage of a first frequency to said first-mentioned and reference motors and second-generating means connected to provide a second a.c. voltage of a second frequency to said first-mentioned and reference motors, the second generating means being variable to provide different frequencies to said first-mentioned and reference motors.
 6. A spectrometer as claimed in claim 5, said output-measuring means comprising a load resistor connected between said first generating means and said bridge-connected first-mentioned and reference motors, and phase-analyzer means connected to said load resistor. 